In recent years, liquid crystal display devices have rapidly come into widespread use for applications such as portable devices, tablets, notebook PCs, and TV sets. This is because the liquid crystal display devices are thin and therefore their space-saving, lightweight, low power consumption, etc. are appreciated. Organic EL display devices have capability of realizing reduction in thickness and power saving at a higher level than that of the liquid crystal display devices, and have high display performance. Therefore the organic EL display devices are rapidly coming into widespread use mainly for smartphones. These display devices are equipped with a phase difference plate for improving their display performance for, e.g., improving viewing angle property and for ensuring visibility under strong external light.
The phase difference plates are broadly classified into two types: a stretched phase difference plate in which a phase difference is developed by stretching a resin to highly orient the resin; and a coated phase difference plate in which a polymerizable liquid crystal is applied onto a substrate serving as a support, oriented to develop a phase difference, and then cured. The stretched phase difference plate has very good workability, and their phase difference, film thickness, etc. can be relatively freely controlled by changing stretching conditions. The coated phase difference plate has a very large birefringence, being about 0.1, and are characterized in that it is highly advantageous to achieve a reduction in thickness as compared to the stretched phase difference plate (Δn=about 0.002).
Recently, the requirement for improvement in display performance has arisen, and phase difference plates are strongly being required to have reverse wavelength dispersion property, and to have precise controllability of the magnitude of the wavelength dispersion. The reverse wavelength dispersion property is a property in which the value of phase difference or birefringence increases as the wavelength λ of light increases from the short wavelength side toward the long wavelength side. The magnitude of wavelength dispersion is a magnitude that indicates a change in the value of phase difference with respect to a change in wavelength and is generally represented by the magnitude of Re(450)/Re(550) and Re(650)/Re(550), where Re(λ) is the value of retardation at a wavelength of λ nm.
With regard to the stretched phase difference plates, there is a proposal described in Patent Literature 1 to meet the aforementioned requirements. According to Patent Literature 1, a phase difference plate having reverse wavelength dispersion property can be provided by stretching a resin prepared by blending or copolymerizing a macromolecular polymer having a positive intrinsic birefringence and a macromolecular polymer having a negative intrinsic birefringence. The magnitude of wavelength dispersion can be controlled by changing the blending ratio of the two macromolecular polymers and the stretching conditions.
With regard to coated phase difference plates, there have been developed a large number of polymerizable liquid crystal materials having reverse wavelength dispersion property, as described in Patent Literature 2. By changing the molecular design of a polymerizable liquid crystal compound with reverse wavelength dispersion, the magnitude of wavelength dispersion can be controlled.
As other methods for controlling the wavelength dispersion in a polymerizable liquid crystal material with reverse wavelength dispersion, there have been proposed methods as in Patent Literatures 3 and 4. In these methods, a polymerizable liquid crystal compound with reverse wavelength dispersion and another polymerizable liquid crystal compound having forward wavelength dispersion property are blended. By controlling the ratio of blending, the wavelength dispersion can be controlled in the direction that causes Re(450)/Re(550) to increase. If the magnitude of wavelength dispersion can be controlled by blending of the polymerizable liquid crystal compounds in such a manner, it is not necessary to return to molecular designing, so that it is possible to rapidly adapt to the requirements from the market.